<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    /* 这个属性对color不管用 */
    input::placeholder-shown {
      color: blue;
      background-color: red;
    }
    /* input::placeholder-shown和::placeholder的区别是什么 */
    input::placeholder {
      color: blue;
      background-color: red;
    }
  </style>
</head>
<body>
  <input type="text" placeholder="我是一个placeholder" />
</body>
</html>